feat(client): add LocalBridge TCP transport and ServerProcess manager

LocalBridge wraps StreamPeerTCP with 4-byte big-endian length-prefix
framing matching server/src/bridge/framing.rs. ServerProcess manages
the Rust server as a subprocess via OS.create_process(). Together
they form the D-020 IPC transport layer for ticket #79.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/protocol/server_process.gd

@@ -0,0 +1,40 @@
+class_name ServerProcess
+## Manages the Rust simulation server as a subprocess (D-020).
+
+var _pid: int = -1
+
+
+## Start the server process. Returns the PID, or -1 on failure.
+func start(server_path: String, args: Array = []) -> int:
+	if _pid > 0 and is_alive():
+		push_warning("ServerProcess: server already running (pid %d)" % _pid)
+		return _pid
+	_pid = OS.create_process(server_path, args)
+	if _pid <= 0:
+		push_error("ServerProcess: failed to start server at %s" % server_path)
+	return _pid
+
+
+## Check if the server process is still running.
+func is_alive() -> bool:
+	if _pid <= 0:
+		return false
+	return OS.is_process_running(_pid)
+
+
+## Stop the server process.
+func stop() -> void:
+	if _pid > 0 and is_alive():
+		OS.kill(_pid)
+	_pid = -1
+
+
+## Get the server PID (-1 if not started).
+func get_pid() -> int:
+	return _pid
+
+
+## Clean up on destruction — kill the server if still running.
+func _notification(what: int) -> void:
+	if what == NOTIFICATION_PREDELETE:
+		stop()